Wheel size and aspect ratio: outer diameter, comfort and what to check

Going up or down in wheel size changes the aspect ratio while keeping the outer diameter about the same. How ride and handling generally change, and what to check besides size — based on information from tyre makers and official documents.

When changing size, start with your vehicle's specified size and the load index.

Load index and inflation pressure

What inch-up and inch-down mean

Both inch-up and inch-down keep the tyre's outer diameter about the same.

Inch-up means moving to a larger rim diameter and a lower aspect ratio while keeping the tyre's outer diameter about the same.

Same outer diameter
Sidewall height

Same outer diameter, different rim diameters: the sidewall height changes

Inch-down is the reverse: a smaller rim diameter with the same outer diameter. See "How to read a tyre size" for the size markings.

How to read a tyre size

Why keep the outer diameter

A different outer diameter can cause speedometer error or contact with the body.

If the outer diameter changes, the speedometer can read differently from your actual speed, and the wheel may touch the body. Speedometer accuracy is set by Japan's vehicle safety standards, and an error beyond the limit can mean failing the shaken inspection.

Compare 225/45 R17 with 225/40 R18.

Sidewall height

225/45 R17: 101.25 mm
225/40 R18: 90 mm

Rim diameter

225/45 R17: 431.8 mm
225/40 R18: 457.2 mm

Outer diameter

225/45 R17: 101.25 mm × 2 + 431.8 mm = 634.3 mm

225/40 R18: 90 mm × 2 + 457.2 mm = 637.2 mm

Difference: +2.9 mm (about +0.46% of 634.3 mm)

Actual outer diameter varies by tyre model.

The illustration compares 225/45 R17 and 225/40 R18: the rim is one inch larger, yet the calculated outer diameter differs by 2.9 mm.

Compare 225/45 R17 with 225/40 R18. Outer diameter is sidewall height × 2 + rim diameter: 225/45 R17 gives 101.25 mm × 2 + 431.8 mm = 634.3 mm, and 225/40 R18 gives 90 mm × 2 + 457.2 mm = 637.2 mm. The rim is one inch larger, but with the lower profile the calculated outer diameter differs by only 2.9 mm (about 0.46%).

These are calculations from nominal sizes. Actual outer diameter varies by tyre model, so check the maker's information. Tyre makers set out the basics: the outer diameter should not differ much from the original size, the wheel must not touch or stick out from the body, and the load index must be equal to or higher than the original; the original size is always the reference. A similar outer diameter alone does not confirm that the wheel fits your vehicle.

Example: 225/45 R17 → 225/40 R18

225/45 R17: 101.25 mm × 2 + 431.8 mm = 634.3 mm
225/40 R18: 90 mm × 2 + 457.2 mm = 637.2 mm
Difference: +2.9 mm (about +0.46% of 634.3 mm)

Comparison by nominal size (calculated)2 rows

Comparison by nominal size (calculated)
Size	Sidewall height	Rim diameter	Outer diameter
225/45 R17	101.25 mm	431.8 mm (17 in)	634.3 mm
225/40 R18	90 mm	457.2 mm (18 in)	637.2 mm

Actual outer diameter varies by tyre model.

Source: Outer diameter formula: Bridgestone tyre size guide

Reading the size marking

For reference: the speedometer rule

30.9–42.5 km/h is the approximate range we calculated from the formula in the safety-standard notice.

For vehicles built on or after 1 January 2007 (excluding motorcycles and some others), the detailed safety-standard notice requires that, when the speedometer shows 40 km/h, the measured actual speed V2 satisfies "10(V1−6)/11 ≦ V2 ≦ (100/94)V1".

10(V1−6)/11 ≦ V2 ≦ (100/94)V1

V1

Speedometer reading: V1 = 40 km/h

Actual speed when the speedometer shows 40 km/h

V2

Actual speed V2: roughly 30.9–42.5 km/h (our approximate calculation)

Applies to vehicles built on or after 1 January 2007 (excluding motorcycles and some others)

Source: MLIT (Japan) road vehicle safety standards, Article 46, and the detailed notice, Article 148

With V1 = 40, that is roughly 30.9 to 42.5 km/h (our calculation).

Diameter tolerance differs by maker

Your vehicle's specified size comes before any tolerance figure.

Makers state outer-diameter tolerances differently.

Michelin Japan, for example, gives a guide of a diameter within -2% to +1.5% and a load index no lower than the original, while still recommending sizes approved by the vehicle maker.

Put your vehicle's specified size first.

Hub Garage store guide

Your vehicle's specified size comes before any tolerance figure.

Load index and inflation pressure

Keep the load index at or above the original, and for XL tyres check the pressure too.

Even when the size changes, the load index must be equal to or higher than that of the original tyre.
XL (extra load) tyres need a different inflation pressure for the same load capacity, so check the correct pressure.

Hub Garage store guide

Keep the load index at or above the original, and for XL tyres check the pressure too.

General effects on ride, noise and handling

Generally, a lower profile gives sharper response and a firmer ride.

In general, a lower-profile tyre has a thinner sidewall: steering response and grip improve, but road bumps come through more, the ride is firmer and noise tends to be louder.

Lower profile

Steering response and grip: better
Road bumps: come through more
Ride: firmer
Noise: louder

Higher profile

Higher-profile tyres are generally considered more comfortable.

Size is not the whole story

Pressure and tread pattern also affect ride and noise.

Ride and quietness do not depend on aspect ratio alone.

Road noise can increase if the pressure is off the correct value, and tread pattern affects pattern noise.
Inch-up can also raise rolling resistance and worsen fuel economy and ride.
We cannot promise how ride or noise will change after a size change.

When choosing wheels

Check rim width, bolt holes and PCD, offset and centre bore together.

Each tyre has an approved rim-width range.
Bolt holes, PCD, offset and centre bore vary by vehicle, and a wheel sticking out past the fender is not permitted.
See "PCD, J, ET and centre bore" for details.

PCD, J, ET and centre bore